1. Field of the Invention
The present invention relates to a fluid infusion pump which peristaltically presses a tube to transfer a fluid in the tube.
2. Description of the Prior Art
A linear peristaltic system fluid infusion pump has its fluid pumping section approximately composed of a groove formed in a wall in which a tube is placed, a plurality of fingers in a stack for peristaltically occluding the tube placed in the groove so that an occluded portion of the tube moves in one direction, and cams for driving the fingers. By advancing and retracting the fingers with respect to the tube while providing a phase shift between adjacent fingers by means of the corresponding cams, the tube is peristaltically occluded sequentially in one direction.
The above-mentioned type of fluid infusion pump is provided with a variety of detection means and control means as follows.
(1) Erroneous tube placement detection
As shown in FIG. 26, pressure sensors 3 are provided on both sides of a cavity 2 in which the aforementioned fingers (not shown) of a fluid pumping section 1 advance and retreat. When a tube is placed in a groove 4, the tube is displaced onto either of the pressure sensors 3, with which a detection value obtained from the pressure sensor 3 varies, to permit detections of the fact that the tube is erroneously placed.
(2) Downstream occlusion detection
By providing a pressure sensor on the downstream side of the fluid pumping section, or by separating the fluid pumping section into an upstream side and a downstream side and providing a pressure sensor at the boundary between the upstream side and the downstream side, a downstream occlusion is detected by a pressure increase on the downstream side (on the fluid delivery side). It is to be noted that the downstream occlusion is detected by a pressure sensor different from the pressure sensor for the erroneous tube placement detection.
(3) Upstream occlusion detection
By providing a pressure sensor on the upstream side of the fluid pumping section, or by separating the fluid pumping section into an upstream side and a downstream side and providing a pressure sensor at the boundary between the upstream side and the downstream side, an upstream occlusion is detected by a pressure increase on the upstream side (on the fluid supply side). It is to be noted that the upstream occlusion is detected by a pressure sensor different from the pressure sensor for the erroneous tube placement detection.
(4) Motor rotational position detection
By providing an encoder comprised of a rotation detecting disk which is provided at an end portion of a shaft around which the cams are mounted and is provided with slits formed at regular intervals and a photosensor, a rotational position with respect to a reference point is detected by means of light travelling through the slits. Otherwise, by driving the shaft by means of a pulse motor, a rotational position with respect to the reference point is detected by the amount of pulses applied to the pulse motor in one rotation of the shaft from the reference point obtained from the photosensor.
When the finger in the lowermost position among the fingers in a stack is pressing the tube, the content volume of the tube does not change, and the outlet of the fluid pumping section is occluded to transfer no medication fluid (dead band). Therefore, it has been performed to preparatorily detect the period of the dead band from the rotational position of the motor detected in a manner as described above, and control the speed of the motor in the period.
(5) Driving torque control (control of current applied to the motor)
A driving torque in a required phase is controlled according to the phase with respect to the reference point detected in the item (4). When controlling the current applied to the motor, the motor input current is reduced in steps. In the case where, when the amount of pulses to be supplied to a stepping motor is set greater than the amount of pulses to be supplied in the slit passage time of the rotation detecting disk, no change occurs in the amount of pulses counted when the slits pass (i.e., slit passage time), it is decided that a stepout (a condition in which the motor is at idle due to a shortage of motor torque) is occurring, and therefore the motor input current is increased.
Unfortunately, there are problems as follows in the detection means and the control means of the aforementioned fluid infusion pump.
(A) Erroneous tube placement detection
Since the erroneous placement of the tube is detected according to the change of the detection value obtained from the pressure sensors 3 provided on both sides of the fluid pumping section 1 as described in regard to the item (1), the pressure sensors 3 are required to have a waterproof structure. Therefore, it is required to provide a sealing section having a thickness slightly greater than 1 mm at each of the pressure sensors 3. Furthermore, it is very difficult to arrange the pressure sensors 3 close to the cavity 2 of the fluid pumping section 1 leaving no space between them, and therefore each of the pressure sensors 3 is placed beside the cavity 2 with a space of about 1 mm provided between them.
Consequently, the pressure sensor is to be provided apart by a space of about 2 mm from the outermost end of the fluid pumping section 1, and therefore a dead zone of about 2 mm from the outermost end of the fluid pumping section 1 takes place. Therefore, when the tube is erroneously placed in a portion apart by about 2 mm from the fluid pumping section 1, it is possible that the erroneous placement of the tube may be not detected.
(B) Downstream/upstream occlusion detection
As described in regard to the items (2) and (3), it is required to provide a special pressure sensor on the downstream side or the upstream side of the fluid pumping section, or at the portion of separation of the fluid pumping section. The above-mentioned arrangement is an obstacle in compacting the fluid infusion pump.
(C) Motor rotational position detection
When the encoder as described in regard to the item (4) is employed, the encoder requires a great amount of volume, which is an obstacle in compacting and reducing the cost of the fluid infusion pump.
Furthermore, when the aforementioned encoder or a pulse motor is employed, the position in which the motor stops is not identified when the power is turned on for the reason that the rotational position is detected from the reference position of one turn of the shaft obtained from the photosensor. Therefore, it is not found which finger is pressing the tube when the power is turned on, and in the worst case, the rotational position cannot be detected until the shaft makes one rotation.
The above-mentioned fact causes a serious problem when the medication fluid is transferred at a low flow rate of 0.1 ml/hr or in a similar case. In detail, a medication fluid to be infused at such a low flow rate of 0.1 ml/hr requires a flow control with high accuracy in most cases. Therefore, the rotating speed of the motor is increased in the period of the dead band to eliminate the period in which no fluid is transferred to thereby allow the fluid to be consistently transferred. However, when the flow rate is 0.1 ml/hr, it takes about one hour to detect the reference position through one rotation of the motor from the time when the power is turned on. Therefore, immediately after the power is turned on, it is possible that a condition in which no medication fluid is transferred may take place.
Furthermore, at a low flow rate of 0.1 ml/hr or in a similar case, there is observed a local change of flow rate as in portions indicated by the arrows in FIG. 27 even when the flow rate is set at a constant value by means of the structure of the fingers and setting of a tube pressing timing of the fingers in the fluid pumping section. It is to be noted that an interval (A) is the dead band when the motor is started.
(D) Driving torque control (control of current applied to the motor)
When controlling the driving torque in the required phase based on the phase with respect to the reference point detected in the item (4), the current cannot be controlled until the motor makes one turn from the time when the power is turned on and detects the reference position (about one hour when the flow rate is 0.1 ml/hr) in the same manner as in the motor rotational position detection. Therefore, it is required to flow a great quantity of current in the above-mentioned time. Consequently, a great current consumption results to require a power battery to have a great capacity, which is an obstacle in compacting the fluid infusion pump.
Furthermore, when controlling the current applied to the motor by means of the encoder as described in regard to the item (5), it is possible that the fluid transfer may be stopped until the slit passage time is counted by supplying pulses more in amount than the amount of pulses to be supplied to the motor in a time in which one slit passes. Therefore, it is possible that a fluid transfer speed required in the case where the fluid is transferred at a low flow rate cannot be controlled with high accuracy.